Gait speed and surface stiffness interactively modulate selected muscle- but not joint-synergy recruitment during compliant-surface walking

Humans adapt walking to different speeds and compliant surfaces, but whether gait speed and surface stiffness shape joint kinematics and muscle activity independently or interactively remains unclear. We reanalyzed an open variable-stiffness treadmill dataset collected at three speeds and four stiffness levels. Tensor decomposition extracted joint and muscle synergies and condition-specific recruitment coefficients. Stride length and stride time were modulated by speed and stiffness without interactions. Joint-synergy recruitment showed speed effects in all three components and stiffness effects in two components, but no speed-by-stiffness interactions. Among five muscle synergies, three were modulated without interaction, whereas two related to weight acceptance and forward propulsion showed significant speed-by-stiffness interactions. Their recruitment increased with speed, but stiffness-dependent differences decreased at higher speeds. These findings suggest that speed and stiffness largely modulate stride- and joint-level control independently, while interactively shaping selected muscle-synergy recruitment.